Add butter and cocoa powder to the the remaining reduced cola in the pan over medium heat until the butter is melted and the cocoa is dissolved.
Remove from heat and cool for 5 minutes.
In a large bowl, whisk together the sugar, flow, baking soda, and salt until incorporated.
Once cooled, add the cola mixture to the flour mixture.
Add the eggs, buttermilk, and vanilla and use and electric mixer on medium until well combined. This will make a thin batter.
Pour the cake batter into the prepared pan.
Bake for ~25-30 minutes or until a probe comes out clean.